The field of the invention relates to optical filters such as solar blind filters, and more particularly to the use of a thallium doped potassium iodide element within a "Barr filter".
A Barr filter is a solid type of band pass filter which includes a suspension of 2,7-dimethyl-diaza-(3,6)-cycloheptadien-(1,6)-perchlorate in a thin film of polyvinyl alcohol.
This suspended compound is referred to in the art as "cation X" and is well known; see Schwarzenbach, Lutz, Helvetia Chima Acta 23, 1139, 1144 (1940).
Previous solar bind filters relied upon the combined effects of several elements to provide definition of the solar blind transmission edge at about 0.28-0.285 microns. The need to use several elements resulted in considerably lower overall in-band filter transmission.